1. Field of the Invention
The present invention provides a direct feed fuel cell for producing electrical energy by electrochemical oxidation/reduction of an organic fuel, and in particular to a direct feed methanol fuel cell with integrated gas separation.
2. The Prior Art
Fuel cell technologies are well known in the art and present opportunities for the commercial development of long-lasting power sources for portable power and electronics applications. With the trend toward greater portability of a wide array of consumer electronics, fuel cell technologies offer promising alternative power sources to meet the increased demand for portable power. Fuel cells can potentially replace or favorably compete with the various types of high density batteries presently used in consumer electronics, such as nickel metal-hydride and lithium ion battery systems, as well as relatively inexpensive alkaline batteries. These types of batteries are less than satisfactory power sources for such consumer electronics as laptop computers and cellular phones either due to their low power density, short cycle life, rechargability or cost. In addition, all these types of batteries present environmental safety concerns and costs for proper disposal.
Fuel cell systems are electricity-generating devices that convert energy produced from a simple electrochemical reaction involving a fuel reactant (methanol or hydrogen) and an oxidizing agent (air or oxygen) into useable electrical energy. “Direct” type of fuel cells, wherein the fuel reactant is directly fed into the fuel cell without prior modification or oxidation, are constructed of an anode electrode, a cathode electrode, and an electrolyte, such as an ion conducting membrane, that separates the electrodes. Fuel reactant is introduced into the fuel cell anode and a catalytic layer intimately in contact with the proton conducting membrane. The catalytic layer acts as an anode electrocatalyst that splits the fuel reactant into protons and electrons as a result of oxidation, releasing hydrogen ions from the reactant molecule. Protons generated at the anode selectively pass through the ion conducting membrane to the fuel cell cathode. A second catalytic layer intimately in contact with the ion conducting membrane acts as a cathode electrocatalyst that reduces hydrogen ions with oxygen molecules provided by circulating air or oxygen to form water. Electrons generated by anodic oxidation of fuel reactant molecules cannot pass through the ion conducting membrane and must flow around the membrane toward the cathode electrode. The flow of electrons is collected by current collection plates on outer sides of the fuel cell and directed into an electrical circuit thereby creating electricity.
Thus, the flow of protons (hydrogen ions) through the ion conducting membrane and the movement of electrons toward the cathode, generate electrical energy in the fuel cell. As long as constant supplies of fuel reactant and an oxidizing agent are maintained, the fuel cell can generate electrical energy continuously and maintain a specific power output. In addition, the fuel cell runs cleanly producing water and carbon dioxide as by-products of the oxidation/reduction of the fuel reactant. Hence, fuel cells can potentially run laptop computers and cellular phones for several days rather than several hours, while reducing or eliminating the hazards and disposal costs associated with high density and alkaline batteries. The challenge is to develop fuel cell technology and to engineer direct fuel cells to meet the form and operation requirements of small-scale or “micro” fuel cells for consumer electronics applications.
Direct fuel cells that demonstrate performance and reliability as potential power systems for portable electronics applications include direct methanol fuel cell (DMFC) systems that employ methanol as the fuel reactant and incorporate an ion conducting membrane electrolyte. Membrane electrolytes are non-liquid, non-corrosive electrolytes capability of operating at low temperatures, which makes such electrolytes commercially attractive for stationary and portable electronics applications. In addition, membrane electrolytes possess excellent electrochemical and mechanical stability, as well as high ionic conductivity that allow them to function as both an electrolyte and a separator.
Prior art direct methanol fuel cells, such as the fuel systems disclosed in U.S. Pat. Nos. 5,992,008, 5,945,231, 5,773,162, 5,599,638, 5,573,866 and 4,420,544, typically employ proton conducting, cation-exchange polymer membranes constructed of a perfluorocarbon sulfonic acid (PFSA) ionomer, such as NAFION® commercially available from E.I. duPont de Nemours and Co. Commercially available NAFION® membranes that act as membrane electrolytes for DMFC systems generally have a thickness of 25 to 175 μm. Composite membranes are also commercially available and can act as membrane electrolytes. Composite membranes are significantly thinner than homogeneous ionomeric membranes and generally have a thickness of 10 to 25 μm. Such composite membranes include, for instance, a polytetrafluorotheylene (PTFE) micromesh material with PFSA-filled pores available from W.L. Gore, Inc. of Newark, Del.
The membrane electrolytes are typically sandwiched between the anode and the cathode electrodes, which are comprised of catalytic layers in intimate contact with surfaces of the membrane electrolyte. The catalytic layers are electrocatalysts that catalyze the electrochemical oxidation/reduction of the fuel reactant, wherein an anode electrocatalyst disassociates hydrogen protons from the fuel reactant and a cathode electrocatalyst effects reduction of hydrogen ions with oxygen to form water. High surface area particles, such as platinum and ruthenium alloy particles, are commonly used as anode electrocatalysts, as disclosed in U.S. Pat. No. 5,523,177. Platinum/ruthenium (Pt/Ru) alloy particles are loaded in a predetermined ratio onto a gas diffusion layer in intimate contact with a surface of the membrane electrolyte to form an anode catalyst layer 42 that acts as the site of electrochemical oxidation. A common cathode electrocatalyst is platinum-black (Pt-black) which is similarly loaded onto a gas diffusion layer in intimate contact with an opposing surface of the membrane electrolyte to form anode catalyst layer 42 that acts as the site of electrochemical reduction. The electrochemical processes in a prior art DMFC system using a Pt/Ru anode electrocatalyst and a Pt-black cathode electrocatalyst are:
Anode:CH3OH + H2O = CO2 + 6H+ + 6eCathode:O2 + 4e + 4H+ = 2H2ONet process:CH3OH + 3/2 O2 = CO2 + 2H2O
The electrocatalysts are typically bonded with or mounted to the gas diffusion layer. The gas diffusion layer is typically constructed of uncatalyzed porous carbon paper or carbon cloth that acts as a gas diffuser and separator. The gas diffusion layer of the anode electrode provides an effective water supply for anodic oxidation of methanol. The gas diffusion layer of the cathode electrode provides an effective supply of oxidizing agent, air or oxygen, while removing water or water vapor from the membrane electrolyte formed from electrochemical reduction of hydrogen ions. The by-products of the electrochemical processes are removed from the fuel cell by an anode vent, exhausting carbon dioxide from the anode electrode, and a cathode vent, removing water and exhausting air from the cathode electrode.
Current collector plates on outer sides of the fuel cell complete the fuel cell unit and conduct and collect electrons generated by the electrochemical oxidation of methanol. Current collector plates are typically constructed of carbon composites or metals, such as stainless steel and titanium, and should exhibit high electronic conductivity. Collector plates should also be impermeable to reactants. Current collector plates may be configured as bipolar plates or include flow fields having a range of flow channel geometries that provide effective supplies of reactant fuel and oxidizing agent, as well as effective removal of air, carbon dioxide and water from the respective electrodes.
DMFC systems are often multi-cell “stacks” comprising a number of single fuel cells joined to form a cell stack to obtain sufficient power densities to meet specific electrical power requirements. The feasibility of using DMFC systems as alternative power sources for portable electronics applications will depend upon the reduction in overall system size, while providing the necessary power densities for electrical power requirements. In addition, DMFC systems for consumer electronics applications will require development and design engineering that will enable methanol fuel cells to self-regulate and generate electrical power under benign operating conditions, including ambient air pressure without active humidification or cooling. Such operating conditions will require the reduction or elimination of auxiliary equipment and external moving parts typically associated with present DMFC systems, such as external fins for heat dissipation, fans for cooling and external flow pumps for supplying pressurized gas reactants and water for sufficient membrane humidification. In addition, peripheral mechanisms or systems, such as pumps and reservoirs used to store and supply methanol fuel and gas separators used to remove gases from liquid fuel cell effluents, will need to be reduced or eliminated in DMFC systems for portable power and consumer electronics applications.
At present, prior art DMFC systems typically operate in two basic configurations, a flow-through configuration and a recirculation configuration, as disclosed in U.S. Pat. Nos. 5,992,008, 5,945,231, 5,795,496, 5,773,162, 5,599,638, 5,573,866 and 4,420,544. The flow-through configuration directly feeds methanol as a vapor or an aqueous stream of either neat methanol or a solution of methanol and water into the anode electrode of the fuel cell. Anodic oxidation by-products, specifically carbon dioxide, as well as fuel impurities and small amounts of unused methanol are removed from the fuel cell through an anode vent. The flow-through configuration has the disadvantages of wasting unused methanol fuel and rendering the fuel supply susceptible to rapid and/or frequent changes in power demands placed on the fuel cell. In addition, the flow-through configuration presents problems with respect to handling the anode effluent discharged from the fuel cell. Peripheral mechanisms or systems are required with the flow-through configuration of DMFC systems to remove and dispose of the anode effluent discharged from the fuel cell. Such mechanism or systems would render flow-through DMFC systems impractical for use in portable electronics applications.
The recirculation configuration of DMFC systems, however, has the advantages of recirculating the anode effluent back into the anode electrode, which conserves unused methanol fuel and contains the anode effluent generated by the electrochemical oxidation/reduction processes at the cost of the power required to circulate the fuel mixture. Referring to FIG. 1, a prior art DMFC system that operates in a recirculation configuration generally includes an external fuel source 2 and a delivery mechanism 3 to supply the anode electrode 5 of the fuel cell 4 with methanol, typically as a methanol and water solution, and an external air source to supply the cathode electrode 6 with air, as an oxidizing agent. The anode effluent contains by-products of the anodic oxidation of methanol, including carbon dioxide and unreacted methanol, while the cathode effluent contains by-products of the cathodic reduction of hydrogen ions and oxygen, including water vapor and air. Gas separators 7, 8 incorporated in effluent return lines are used to remove gases from effluent fluids. The gas separator 7 incorporated in an anode effluent return line effectively separates carbon dioxide from the unused methanol solution and exhausts carbon dioxide from the DMFC system. Similarly, the gas separator 8 incorporated in the cathode effluent return line separates air from water vapor and exhausts carbon dioxide from the DMFC system, allowing water to be returned to the fuel delivery mechanism 3.
Prior art DMFC systems with recirculation configurations overcome the problems of handling anode effluent, conserving unused methanol fuel and rendering the fuel supply impervious to rapid changes in power demands of the fuel cell. Such features are highly advantageous for use of DMFC systems in portable power supplies and portable consumer electronics. However, recirculation configurations of prior art DMFC systems must incorporate auxiliary or external peripheral equipment in the recirculation loops, specifically gas separators, that render recirculating DMFC systems less feasible for portable power and electronics applications.
Therefore, it would be desirable to provide a recirculating direct feed methanol fuel cell system, wherein external gas separators are eliminated from the recirculation loops and by-product gases are removed from liquid streams within the fuel cell system.